Respiratory syncytial virus (RSV) is a member of the genus Pneumovirus of the family Paramyxoviridae. Human RSV (HRSV) is the leading cause of severe lower respiratory tract disease in young children and is responsible for considerable morbidity and mortality in humans. RSV is also recognized as an important agent of disease in immunocompromised adults and in the elderly. Due to incomplete resistance to RSV in the infected host after a natural infection, RSV may infect multiple times during childhood and adult life.
This virus has a genome comprised of a single strand negative-sense RNA, which is tightly associated with viral protein to form the nucleocapsid. The viral envelope is composed of a plasma membrane derived lipid bilayer that contains virally encoded structural proteins. A viral polymerase is packaged with the virion and transcribes genomic RNA into mRNA. The RSV genome encodes three transmembrane structural proteins, F, G, and SH, two matrix proteins, M and M2, three nucleocapsid proteins N, P, and L, and two nonstructural proteins, NS1 and NS2.
Fusion of HRSV and cell membranes is thought to occur at the cell surface and is a necessary step for the transfer of viral ribonucleoprotein into the cell cytoplasm during the early stages of infection. This process is mediated by the fusion (F) protein, which also promotes fusion of the membrane of infected cells with that of adjacent cells to form a characteristic syncytia, which is both a prominent cytopathic effect and an additional mechanism of viral spread. Accordingly, neutralization of fusion activity is important in host immunity. Indeed, monoclonal antibodies developed against the F protein have been shown to neutralize virus infectivity and inhibit membrane fusion (Calder et al., 2000, Virology 271: 122-131).
The F protein of RSV shares structural features and limited, but significant amino acid sequence identity with F glycoproteins of other paramyxoviruses. It is synthesized as an inactive precursor of 574 amino acids (F0) that is cotranslationally glycosylated on asparagines in the endoplasmic reticulum, where it assembles into homo-oligomers. Before reaching the cell surface, the F0 precursor is cleaved by a protease into F2 from the N terminus and F1 from the C terminus. The F2 and F1 chains remains covalently linked by one or more disulfide bonds.
Immunoaffinity purified full-length F proteins have been found to accumulate in the form of micelles (also characterized as rosettes), similar to those observed with other full-length virus membrane glycoproteins (Wrigley et al., 1986, in Electron Microscopy of Proteins, Vol 5, p. 103-163, Academic Press, London). Under electron microscopy, the molecules in the rosettes appear either as inverted cone-shaped rods (˜70%) or lollipop-shaped (˜30%) structures with their wider ends projecting away from the centers of the rosettes. The rod conformational state is associated with an F glycoprotein in the pre-fusion inactivate state while the lollipop conformational state is associated with an F glycoprotein in the post-fusion, active state.
Electron micrography can be used to distinguish between the prefusion and postfusion (alternatively designated prefusogenic and fusogenic) conformations, as demonstrated by Calder et al., 2000, Virology 271:122-131. The prefusion conformation can also be distinguished from the fusogenic (postfusion) conformation by liposome association assays. Additionally, prefusion and fusogenic conformations can be distinguished using antibodies (e.g., monoclonal antibodies) that specifically recognize conformation epitopes present on one or the other of the prefusion or fusogenic form of the RSV F protein, but not on the other form. Such conformation epitopes can be due to preferential exposure of an antigenic determinant on the surface of the molecule. Alternatively, conformational epitopes can arise from the juxtaposition of amino acids that are non-contiguous in the linear polypeptide.
It has been shown previously that the F precursor is cleaved at two sites (site I, after residue 109 and site II, after residue 136), both preceded by motifs recognized by furin-like proteases. Site II is adjacent to a fusion peptide, and cleavage of the F protein at both sites is needed for membrane fusion (Gonzalez-Reyes et al., 2001, PNAS 98(17): 9859-9864). When cleavage is completed at both sites, it is believed that there is a transition from cone-shaped to lollipop-shaped rods.